A person can suffer decompression sickness (DCS), also called decompression illness (DCI), if the person is exposed to a breathing gas with a pressure that decreases too quickly. DCS can be mild or severe and can have various neurological and audiovestibular manifestations and symptoms such as skin rash, pain, paralysis, blindness, and even death.
For instance, DCS can occur when a diver ascends to the water surface too quickly at the end of a dive. Both the depth and duration of the dive can influence the likelihood that DCS will occur. During a dive, a diver typically uses a breathing gas that contains oxygen and an inert gas such as nitrogen, with the total pressure of the breathing gas regulated to match the hydrostatic pressure at the current depth.
Some of the inert gas in the breathing gas can be absorbed by and dissolved in body tissues when the person is exposed to the breathing gas. The concentration of inert gas dissolved in a body tissue is dependent on the inert gas partial pressure in the breathing gas, referred to as the ambient inert gas partial pressure (Pa,n), and the length of exposure to the breathing gas. For a given Pa,n, the concentration of inert gas dissolved in the body tissue at equilibrium is referred to as the saturation concentration. The higher the Pa,n, the higher the saturation concentration. If, for a given Pa,n (such as at a particular depth in a dive), the body tissue is under-saturated, i.e., having a lower dissolved inert gas concentration than the saturation concentration, more inert gas will be dissolved. On the other hand, if the body tissue is super-saturated, i.e., having a higher dissolved inert gas concentration than the saturation concentration, some of the dissolved inert gas will be released from the tissue.
For example, when a diver descends in water, Pa,n and the saturation concentration increase; when the diver ascends, Pa,n and the saturation concentration decrease. The period in a dive when the body tissues absorb inert gas is referred to as the “compression” phase of the dive. This typically includes the descent portion of the dive and the period when the diver is at the deepest depth prior to saturation. Saturation can be reached when the diver stays at a depth for a long period of time, or ascends from a deeper to a shallower depth. Supersaturation can also occur when the diver ascends. The period during or after a dive when the body tissues are super-saturated is referred to as the “decompression” phase. DCS can occur during the decompression phase. The rate at which inert gas is released from a body tissue at any given time during the decompression phase may depend on the difference between the values of the concentration of inert gas actually dissolved in the body tissue and the saturation concentration in that tissue at that time. The greater the difference, the faster the release rate. If the release rate is too fast, DCS can occur.
Therefore, it is important that divers follow safe dive profiles or decompression schedules to avoid DCS. In simple terms, a dive profile is a representation of the depth or ambient pressure (Pa), as a function of time during a dive. A dive profile can be presented in the form of a line graph, a chart, or a table. To avoid DCS, a diver can ascend continuously at a sufficiently slow rate. Alternatively, a diver can ascend relatively rapidly but in stages, pausing or stopping at each of one or more progressively shallower depth(s) for a certain time period during ascent (known as “decompression stops”). A further alternative is to start the ascent before a decompression stop becomes necessary (such a dive is known as a “no decompression” or “no-stop” dive). The maximum bottom time for a no-stop dive is referred to as the No Decompression Limit (NDL). The NDL can also be expressed as the remaining safe bottom time for a no-stop dive.
Dive tools, such as decompression tables, dive wheels, and dive computers, have been used to assist divers for preventing DCS. A diver can use a dive tool to determine the NDL during a dive, or, a safe decompression schedule if the NDL has been exceeded.
The NDL or the decompression schedules can be determined based on the risks of DCS for different dive profiles. The risk of DCS for a given dive can be assessed from Pa, Pa,n, and the concentrations of dissolved inert gas in various body tissues. The values of Pa during a dive can be readily measured. The values of Pa,n during a dive can be derived from Pa for a given breathing gas. However, there is no practical and convenient way of measuring the concentrations of inert gas dissolved in various body tissues of the diver.
Therefore, conventionally, the risks of DCS are typically assessed based on decompression models. A conventional decompression model is a mathematical model with two distinct components. One component is a gas distribution model that describes the distribution of inert gas in the body tissues at all times. The other component is a risk function that relates the risk of DCS to the degree of inert gas supersaturation in the body tissues. Examples of decompression models include the Haldane model, the Reduced Gradient Bubble Model, the Varying Permeability Model, the Linear-Exponential model, and the like.
In conventional decompression models, such as the Haldanian models, the human body is represented as a number of parallel compartments (PC) each connected to the bloodstream. Each compartment exchanges gas with the bloodstream but is otherwise isolated from the other compartments. Each compartment has a characteristic tissue halftime and different compartments have different halftimes. For example, in the original Haldane model, there are five compartments with respective halftimes of 5, 10, 20, 40 and 75 minutes; a model used by the U.S. Navy (USN) has six compartments with respective halftimes of 5, 10, 20, 40, 80, and 120 minutes. In these models, the rate of gas uptake and release for each compartment is dependent on its halftime. For a given dive profile, the contribution to the risk of decompression sickness made by each compartment can be calculated. The overall risk at any given time takes into account contributions to the risk from all the compartments. For example, for a parallel N-compartment gas distribution model with a continuous risk function, the overall instantaneous risk r(t) at time t can be expressed as:
                                          r            ⁡                          (              t              )                                =                                    ∑                              i                =                1                            N                        ⁢                                          r                i                            ⁡                              (                t                )                                                    ,                            (        1        )            where ri(t) is the instantaneous risk of decompression sickness, per unit time, for the ith compartment at time t. The total risk, which is related to the probability of DCS for the entire dive profile, can be assessed by taking into account r(t) over all the decompression components of the profile.
However, PC-based decompression models, on which many existing dive tools are based, have shortcomings. One problem is that these decompression models do not accurately represent the human body's actual response to decompression stress over a wide range of exposure profiles. Consequently, predictions derived from these models are inaccurate in many situations. A dive tool based on one of these models cannot provide good predictions for a wide range of dive profiles, and thus has limited application. A conventional dive tool may underestimate the risks of DCS for certain types of dive profiles and its users may unknowingly take unacceptable risks. To compensate for the inaccuracy of these models, some conventional dive tools such as dive computers use model parameters that are selected conservatively to avoid underestimating the risks of DCS for various types of dive profiles. As a result, for some types of dive profiles, these dive tools unnecessarily overestimate the risk. Therefore, divers, particularly recreational divers, who use these dive tools often have to shorten the bottom time or lengthen the stop time during ascent unnecessarily.
Accordingly, there is a need for methods and devices for predicting risks of DCS, which are based on decompression models that can provide a more accurate representation of the human body's response to decompression stress, or can provide satisfactory predictions over a wide range of exposure profiles.